This invention relates to molten carbonate fuel cells and, in particular, to cathode side hardware employed in such cells.
As used herein the term “cathode side hardware” is defined as the current collector and/or the bipolar plate on the cathode side of a fuel cell and, in particular, a molten carbonate fuel cell. Corrosion is a life-limiting factor for molten carbonate fuel cells. The prevailing corrosion is at the oxide-gas (or liquid) interface, i.e., at the cathode side hardware. This hardware is typically formed from chromium containing stainless steel and corrosion of the hardware is governed by the outward cation diffusion via metal vacancies. It is estimated that twenty five percent (25%) of the internal resistance of a molten carbonate fuel cell could be attributed to the oxide corrosion layer that forms on the cathode side hardware.
More particularly, the cathode current collector, generally made of 316L stainless steel, becomes corroded during fuel cell operation and multi-corrosion oxide layers having a relatively high electrical resistance are formed on the surface of the collector. Moreover, the formed corrosion layers usually thicken with time.
Additionally, the corrosion layers on the cathode side hardware cause electrolyte loss through surface and corrosion creepage. Electrolyte surface creepage is controlled by capillary forces dominated by the surface roughness, porosity and pore size in corrosion layers. Electrolyte corrosion creepage is controlled by scale thickness and phase composition of the formed scale. In cathode side hardware formed with stainless steel, a high roughness of the scale surface and the porous structure of the scale cause high electrolyte surface creepage.
It has been estimated that electrolyte loss in a molten carbonate fuel cell is a significant life-limiting factor for achieving a lifetime of 40,000 hours. Analysis of cathode side hardware has indicated that sixty five percent (65%) of electrolyte loss is attributed to this hardware. It is estimated that a forty five percent (45%) reduction in electrolyte loss could result in ˜1.7 yr life extension of the molten carbonate fuel cell.
In order to counter the corrosion of the cathode side hardware, it has been proposed to provide a protective oxide coating on the cathode side hardware to realize a low contact resistance and low electrolyte loss. These coatings, however, must satisfy stringent requirements in that they must, on the one hand, have a high corrosion resistance, and, on the other hand, a high electrical conductivity. The coatings must also be able to provide a stable surface oxide capable of providing a barrier between the coating alloys and the environment of the molten carbonate fuel cell.
U.S. Pat. No. 5,643,690 discloses a coating of this type in the form of a non-stoichiometric composite oxide layer (Ni ferrite based oxide) formed by in cell oxidation of a layer of Fe, Ni and Cr clad on cathode current collector. Similarly Japanese patent 5 -324460 discloses a stainless steel collector plate covered with a NiO layer (formed by oxidation of a Ni layer plated or clad on a cathode current collector). The coatings formed in these cases are porous and consume a significant amount of electrolyte. Also, the electrical conductivity of the layers may not be as high as desired.
U.S. patent application Publication No. US 2002/0164522, assigned to the same assignee hereof, discloses another coating layer which is formed as a conductive layer of ceramic material using a sol-gel process. The materials used for the conductive layer in this case are, preferably, LiCoO2 or Co doped LiFeO2, and the thickness of the layer is between 1 to 5 μm.
The aforesaid conductive ceramic layers of the 2002/0164522 publication have proven satisfactory in providing corrosion resistance of the cathode side hardware. However, the materials are costly and add to the overall expense of the fuel cell. Moreover, higher conductivities are still desired. Fuel cell designers have thus continued to search for other coating materials which offer the desired corrosion resistance, but are more cost effective and are higher in conductivity.
It is therefore an object of the present invention to provide cathode side hardware which does not suffer from the above disadvantages; and
It is a further object of the present invention to provide cathode side hardware having a high corrosion resistance and electrical conductivity and a lower cost.